1. Field of the Invention
This invention relates to an improved chemical reactor, especially a polymerization reactor. In particular, it relates to a novel, improved back-mixed chemical reactor useful in the production of butyl rubber.
2. Background
Reactors are of various designs, the form and shape thereof depending largely on the nature of the reaction to be conducted therein. In conducting polymerization or condensation reactions where liquid chemical raw materials are catalytically converted into elastomeric solids or semi-solids, as in the production of synthetic rubber from low boiling hydrocarbons, a reaction mixture is circulated as a slurry in a reactor into which reactants and catalysts are injected, and product withdrawn. Where, e.g., isobutylene is polymerized with a diolefin in the presence of a Friedel-Crafts type catalyst, e.g., an aluminum halide catalyst, dissolved in a diluent of low freezing point, i.e., at temperatures of about -100.degree. F. to -160.degree. F. to produce butyl rubber, a back-mixed reactor is employed; typically a one-tube pass system as described by reference to U.S. Pat. No. 2,474,592. Such reactor is characterized generally as a vertically oriented elongate vessel formed by an enclosing side wall within which is provided an axially mounted draft tube of relatively large diameter surrounded by a relatively large number of small diameter tubes which extend downwardly from an upper common plane to a lower common plane where the upper and lower terminal ends of the small diameter tubes and draft tube, respectively, terminate. An axial flow pump, provided with a rotating impeller, which extends into the draft tube within which it is partially housed, is located in the bottom of the reactor to maintain the reaction mixture in a well dispersed state, and pump same up the draft tube; the reaction mixture including the diluent, catalyst, and reactants which are directly introduced into the bottom of the reactor, and a portion of the reaction mixture which after upward transport through the draft tube is recycled from the top of the reactor downwardly through the small diameter tubes which surround the draft tube. The outer walls of the reaction vessel form a jacket through which a liquid hydrocarbon coolant is circulated to remove the exothermic heat of reaction via heat exchange contact with the outer walls of the small diameter tubes, and wall of the central draft tube.
Whereas this reactor has been commercially used by the industry for many years for conducting these types of reactions, the reactor is nonetheless far less efficient than desirable. For example, vortex "whirl" at the impeller exit, or cavitation bubbles on the impeller, or both, impairs the hydraulic efficiency of the pump to a level of about fifty percent of that which is theoretically possible. This results in higher temperature surfaces throughout the reactor and increased heat duty for the reactor. For best operation, it is essential that the temperature of a butyl reactor, due to the high temperature sensitivity of the butyl polymerization process be maintained between about -130.degree. F. and -145.degree. F., and as uniform as possible.
Polymer fouling is another serious problem encountered in this type of reactor. Polymer deposits upon and fouls heat transfer surfaces within the reaction vessel; the polymer adhering tenaciously to the metal surfaces as a continuous film, and in severe cases as large masses of rubber. The reason, or reasons, for this phenomenon is not well understood though, it is known that mass fouling is caused by local overheating. Nonetheless, polymer fouling presents a serious problem and it has greatly limited the usefulness, as well as the efficiency of this type of reactor. For example, it is reported in U.S. Pat. No. 2,999,084 that "--Commercial experience has demonstrated that mass fouling is a limiting factor of prime importance with respect to the rate of production of tertiary isoolefin polymers in that fouling to an extent sufficient to inhibit adequate refrigeration will occur at erratic and unpredictable intervals within the range of about 10 to 90 hours"; and that "--When this happens, it is necessary to `kill` the reaction medium and clean out the reactor before resuming the polymerization reaction," this normally requiring 10 to 20 hours. At its best, in any event, polymer fouling results in poor heat transfer, and loss of efficiency in the process operation. At its worse, the usefulness of the reactor is greatly curtailed.
For these reasons there presently exists a need for a new, novel, or improved reactor; particularly a reactor wherein the components of the reaction mixture are better dispersed, there is less polymer fouling of the reactor, and better hydraulic and thermal efficiency in the operation of the reactor.